Five subjects that no human should show arousal towards or try any sexual acts with. Once you break one rule, you break them all. These are;
1. Animals
2. Men (presuming you're male)
3. Family
4. Children
5. Dead People

Sean: Tom's getting off a 14 year old again.
Matthew: Lock up your cat, he'll be going for that next!
Sean: What do you mean?
Matthew: 'The Five Rules', once you break one rule you break them all...